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PICList Thread
'[EE] Unusual H-bridge'
2011\10\27@040029 by IVP

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Hi all,

I came across this H-bridge circuit, a design I've not seen before

The only comment is "Note the diode; this allows adequate base current
from a regulated 5V supply. Have an LED if you are putting in the full
7.4V as an input, such as with booster transistors"

Apparently it "works well with RC bots", such as this tracked tank

Thoughts ? My first one was the losses in the Darlingtons

Joe

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part 3 17172 bytes content-type:image/jpeg; name="tank.jpg" (decode)


part 4 181 bytes content-type:text/plain; name="ATT00001.txt"
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2011\10\27@042612 by Wouter van Ooijen

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> Apparently it "works well with RC bots", such as this tracked tank

1. no freewheeling diodes?

3. which component determines the base currents? the impedance of the driving source, or the ohmic impedance component of the LEDs??

3. what whacky way to draw a diagram! did he use word?

4. a typical 'it works on my desktop' circuit. If I found myself working for a company that put this in a product I would run, emigrate, and get a new name.

5. over which voltage range does this work? and for which brands and colors of LEDs? and over which temperature range?

--
Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

2011\10\27@044349 by IVP

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> 1. no freewheeling diodes?

Small DC motors can be very noisy electrically. I'd be wary of EMF
interference to nearby logic

> 3. which component determines the base currents? the impedance of
> the driving source, or the ohmic impedance component of the LEDs??

As it was (IIRC, I saw it a couple of weeks ago) using a PICAXE,
then the max source/sink would be that of whatever PICs are used
as PICAXEs, 20-25mA. I guess that would be why Darlingtons

> 5. over which voltage range does this work? and for which brands and
> colors of LEDs? and over which temperature range?

Presumably a 6-pack of NiCds, and green (as can be seen in the photo

2011\10\27@065121 by Wouter van Ooijen

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>> 1. no freewheeling diodes?
>
> Small DC motors can be very noisy electrically. I'd be wary of EMF
> interference to nearby logic\

I'd be aware of *damaging* nearby chips!

>> 3. which component determines the base currents? the impedance of
>> the driving source, or the ohmic impedance component of the LEDs??
>
> As it was (IIRC, I saw it a couple of weeks ago) using a PICAXE,
> then the max source/sink would be that of whatever PICs are used
> as PICAXEs, 20-25mA. I guess that would be why Darlingtons

for the first stage (and even then it would be very bad practice). but the second stage is slaved to the low-impedance output of the first.

>> 5. over which voltage range does this work? and for which brands and
>> colors of LEDs? and over which temperature range?
>
> Presumably a 6-pack of NiCds, and green (as can be seen in the photo)

Did he test it during other phases of the moon?

--
Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

2011\10\27@074314 by Bob Axtell

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On 10/27/2011 1:26 AM, Wouter van Ooijen wrote:
>> Apparently it "works well with RC bots", such as this tracked tank
> 1. no freewheeling diodes?
>
> 3. which component determines the base currents? the impedance of the
> driving source, or the ohmic impedance component of the LEDs??
>
> 3. what whacky way to draw a diagram! did he use word?
>
> 4. a typical 'it works on my desktop' circuit. If I found myself working
> for a company that put this in a product I would run, emigrate, and get
> a new name.
>
> 5. over which voltage range does this work? and for which brands and
> colors of LEDs? and over which temperature range?
>
Too vague for me. No thanks.

--Bo

2011\10\27@085706 by Kerry Wentworth

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IVP wrote:
> Hi all,
>
> I came across this H-bridge circuit, a design I've not seen before
>
> The only comment is "Note the diode; this allows adequate base current
> from a regulated 5V supply. Have an LED if you are putting in the full
> 7.4V as an input, such as with booster transistors"
>
> Apparently it "works well with RC bots", such as this tracked tank
>
> Thoughts ? My first one was the losses in the Darlingtons
>
> Joe
>

My first thought was that the diodes are backwards.

Kerry

> ------------------------------------------------------------------------
>
>

2011\10\27@132653 by Bob Blick

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On Thursday, October 27, 2011 10:26 AM, "Wouter van Ooijen"  wrote:

> 4. a typical 'it works on my desktop' circuit. If I found myself working
> for a company that put this in a product I would run, emigrate, and get
> a new name.

Best comment I've seen in over a week, I'm in total agreement with you.

Cheerful regards, Bob

-- http://www.fastmail.fm - A fast, anti-spam email service.

2011\10\27@134041 by Dwayne Reid

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At 02:00 AM 10/27/2011, IVP wrote:
>Hi all,
>
>I came across this H-bridge circuit, a design I've not seen before
>
>The only comment is "Note the diode; this allows adequate base current
>from a regulated 5V supply. Have an LED if you are putting in the full
>7.4V as an input, such as with booster transistors"
>
>Apparently it "works well with RC bots", such as this tracked tank

Aside from the diodes being drawn backwards, I think that its pretty cute.  I don't think that this is anything that I'd ever use (most of my stuff is for Industrial customers) but I can sure see a high-volume toy manufacturer going with something like this.

I'm assuming that they are using older (low-intensity) green or yellow LEDs (Vf ~2.1V)     as opposed to red (~1.7V) or high-intensity green, blue or white LEDs (~3.2V).

Let me see . . . You said Darlington transistors.  7.4V - 1.2V (Vbe) - 2.5V (nominal idle point) = 3.7V.  Hmm - use 2- red LEDs in series.

I'm assuming that in China, cheap LEDs can be had for about the same price as I would pay for a resistor here in North America.  That would make this a very cost effective design.  Especially if the LEDs were part of the exterior 'bells and whistles' jazzy lighting effects.

Not a great design, in my opinion, but entirely adequate for the consumer toy market.

In fact, the more I study the diagram and work the numbers, the more I think that its a pretty good design.

I'd probably add a single series resistor between the W2 output and the center of the LED string that feeds the upper half-bridge.  You could also probably change the upper half-bridge transistors to be not Darlington if you wished to reduce losses in the output transistors.

dwayne

-- Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2011\10\27@152350 by Dwayne Reid

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At 11:26 AM 10/27/2011, Bob Blick wrote:
>On Thursday, October 27, 2011 10:26 AM, "Wouter van Ooijen"  wrote:
>
> > 4. a typical 'it works on my desktop' circuit. If I found myself working
> > for a company that put this in a product I would run, emigrate, and get
> > a new name.
>
>Best comment I've seen in over a week, I'm in total agreement with you.

I first agreed with you, but taking a much closer look at the circuit reveals that it is actually a pretty cool design, given the stated working conditions.

1) Vdd = 7.4Vdc.  6 NiCd or NiMh cells have a reasonably flat discharge curve - by the time the battery voltage decays to around 7V, there is only a few percent of charge left in the cells.

2) Vcontrol = 0V, float, 5V.  These are nicely defined voltage levels that actually do as they are supposed to do.

My only change would be to add a resistor from the output of the bottom half-bridge to the center of the LED chain that controls the top chain instead of the direct connection that is currently there.

As I mentioned in my previous post, this is something that I would not design myself.  But I can't really fault the design - it does exactly what it is supposed to do at very low cost under the very specific conditions that were stated (Vdd = 7.2 - 7.4Vdc).  Don't ever try to use it with any other supply voltage - it either just won't work or it will generate smoke.

dwayne

-- Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

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